In examining the detection of increments and decrements in intensity,
it is important to distinguish between two different methods that can
have very different results: "gated" tones vs. "ongoing" tones. In
the gated-tones method used in the Sinnott et al. (1985) paper, a
standard 250 ms tone was presented twice per second for three seconds
(i.e., six repetitions) and then a comparison tone of the same
duration was presented in the same repeating configuration. The
question was whether or not the standard and comparison tones had the
same intensity. In the gated configuration, the difference in
intensity as measured by neural firing rate is probably a pretty good
model of the information underlying the decision. For this reason,
increments and decrements are expected to have similar detectability,
and this is what Sinnott found, at least for humans.

The ongoing-tone paradigm, in which differences between increments
and decrements are found more often, involves the presentation of a
continuous or long-duration tone into which a change in intensity is
introduced. In this stimulus, the decrement is subject to forward
masking, whereas the increment is not, and so differences might be
expected. In my dissertation (Gallun and Hafter, 2006), I suggested
that changes in ongoing tones could be detected by modulation-
sensitive mechanisms. In such a mechanism, forward masking would
result in reduced modulation depth for decrements, and thus higher
thresholds. Such mechanisms would be less able to detect changes in
tones gated on and off in the manner used by Sinnott. Consistent
with this difference in mechanism is the difference in JNDs for the
two methods, with ongoing tones yielding JNDs about half the size of
those for gated tones. The duration and number of the changes will
also influence the amount of forward masking and thus the difference
between increments and decrements. For more details and references,
please feel free to contact me off-list.

Is there anything known about the existence of differences in the=20
sensitivity to intensity increments versus to intensity decrements?

Laurent Demany pointed me to a paper by Sinnott et al. (1985) who
found=20
no such difference in humans, while they found an advantage for=20
increments in monkeys:
Sinnott, J. M., Petersen M. R., Hopp, S. L. (1985).
Frequency and intensity discrimination in humans and monkeys.
Journal of the Acoustical Society of America, 78, 1977-1985.
Is this finding (as to the symmetry of human increment / decrement=20
sensitivity) unchallenged?